1. Field of the Invention
The present invention relates to a broad-band exchange system for selectively connecting broad-band signal sources to subscribers who receive broad-band signals, via a broad-band switching arrangement in which the cross point circuits, whose function is to selectively connect input lines to output lines within the switching arrangement, can be formed by logic linking elements constructed in ECL technology in order to achieve as short as possible switching times and in view of the signals which are to be switched through, are preferably pulse modulated and possess signal frequencies of, for example, up to a few hundered MHz (see Pfannschmidt: "Arbeitsgeschwindigkeitsgrenzen von Koppelnetzwerken fur Breitband-Digitalsignale", Diss. TU Bswg. 1978 and the German allowed application No. 28 28 662).
2. Description of the Prior Art
A broad-band switching arrangement of the type generally set forth above can serve, for example, to switch television programs to subscribers desirous of receiving such programs, as provided by more recent developments in telecommunications technology which have lead to broad-band communications networks in which subscriber lines are formed by light wave guides, wherein, in the subscriber plane, a star network composed of light wave guides with one light wave guide provided for each dwelling unit has proved favorable, which connects the subscriber's dwelling to a broad-band exchange (which is advantageously spatially united with the next telephone exchange), and by way of which all of the telecommunications services relating to the dwelling unit in question are handled, wherein at least the following communications possibilities should be taken into account for a residential terminal which will prove reliable in the future:
three television channels for three television receivers having independent access to all of the television signal sources which can be reached from the exchange and to all of the television programs available in the exchange; PA0 three UKW audio radio channels (stereo); PA0 digital telephony; PA0 screen text; PA0 video telephone; and PA0 return channels for program selection and possibly for the handling of interactive services.
In a broad-band signal exchange system of this kind, not only must one and the same signal source be simultaneously connected to a plurality of subscriber terminals, but, vice-versa it must also be possible to connect one and the same subscriber terminal simultaneously to a plurality of signal sources.
For this purpose, it can be provided that the (pulse modulated) signals of the individual signal sources are multiply supplied to the input, which is individually assigned to a signal source, of a broad-band switching arrangement comprising crosspoint circuits designed in ECL technology, in each case by TDM, in a plurality of time channels, corresponding in number to the signal sources which can be simultaneously connected to one and the same subscriber, within a TDM system which comprises at the least these time channels, and in the broad-band switching arrangement the subscriber-individual outputs are each connected in time channels, individually assigned to the subscribers, from amongst the aforementioned plurality of time channels, to the desired signal source (see German Pat. No. 29 44 784).
A broad-band switching arrangement of this kind, wherein the crosspoint switches can be formed by ECL logic linking elements, can be used to transmit pulse modulated signals, for example PPM signals, of large band width.
However, broad-band signal switching is dependent not only upon broad-band characteristics or high-speed of a crosspoint switch, which in the case of a design using ECL logic linking elements also follows signal pulses having edge rise and fall times in the order of 1 ns, but also by the blocking attenuation (which exists when the cross point switch is in the blocked state) of such signal pulses. This is to say that the crosspoint switches possess only a limited blocking attenuation, thus current ECL crosspoint switches for signal bandwidths of approximately a hundred MHz can possess a blocking attenuation of approximately 40 dB, so that spatial cross talk can occur between (adjacent) transmission channels belonging to the same time slot and possibly also to time-related cross talk between transmission channels (which employ the same switch-through paths of adjacent time slots). With a blocking attenuation of approximately 40 dB, for example in the case of television transmission, this can lead to a visible cross talk between transmission channels belonging to the same time slot.